limb darkening

A darkening of the solar limb at optical wavelengths,
arising because the observer's line of sight passes through shallower, cooler,
and more absorbing layers of the photosphere
as it moves away from the center of the Sun's disk; a similar effect occurs
with Jupiter and with the members of eclipsing
stars as they move in front of each other, and has been detected in
the case of several single stars, including Hamal.
Limb brightening is a brightening of the solar limb at X-ray, extreme ultraviolet,
and radio wavelengths, due to the observer's line of sight intercepting
more of the hot inner corona and less of
the cooler photosphere as it moves away from the center of the Sun's disk.